1. Field of the Invention
This invention relates to improvements in a process and a system for treating a treatable material such as waste containing a large amount of noxious components such as halogen substances and sulfur, under a heat treatment, in order to prevent generation of virulently poisonous dioxins and make waste gas and the treatable material harmless.
2. Description of the Prior Art
General waste such as urban waste (trash), industrial waste, shredder dust, and plastic waste such as polyvinyl chloride contain a large amount of halogen substances (chlorine, bromine, iodine, fluorine, and astatine), particularly chlorine. Accordingly, when such waste or the like is subjected to heat treatment such as incineration, a large amount of noxious chlorine-containing gas such as hydrogen chloride gas and chlorine gas is generated and becomes a cause for producing virulently poisonous dioxins in residue obtained after the incineration and in flying ash contained in waste gas. Such incineration treatment has been made also on waste (such as waste tire) containing sulfur, in which a large amount of sulfur oxides gas (SOx) are generated. Therefore, treatment for such sulfur oxides gas has been accomplished.
In order to remove the above noxious gases, it has been proposed to spray alkali material (such as lime powder) into an incineration furnace supplied with waste (the treatable material), as disclosed in Japanese Patent Provisional Publication No. 54-93864. In this proposition, the sprayed alkali material reacts with chlorine-containing gas generated under incineration of the waste, thereby to form harmless chloride (such as calcium chloride) thus making waste gas harmless.
It has been also proposed that waste is incinerated upon adding calcium-containing alkali material such as lime (CaCO3) or slaked lime (Ca(OH)2), or that the alkali material is filled in a filter through which waste gas is passed, thus removing noxious chloride gas or sulfur oxides gas from the waste gas. Such propositions are disclosed in Japanese Patent Publication No. 2-10341, Japanese Patent Provisional Publication No. 1-296007, and Japanese Patent Provisional Publication No. 59-12733.
It will be understood that the above conventional techniques are intended to once allow noxious gas to be generated, and thereafter to remove the noxious gas.
Furthermore, it has been known that the treatable material is thermally decomposed or subjected to dry distillation, and thereafter residue formed upon thermal decomposition is reduced in volume under carbonization, ashing or the like. One of such treatments is carried out as follows: The treatable material is thermally decomposed in a single rotatable furnace (rotary kiln). Residue discharged from the rotatable furnace is thereafter incinerated in a stoker. Thermal decomposition gas from the rotatable furnace is burned in a re-burning chamber, generating high temperature gas. The high temperature gas (waste gas) is passed through a boiler or the like and then introduced into a reaction tower into which slaked lime slurry is sprayed so as to react with the waste gas. This process is disclosed in Japanese Patent Provisional Publication No. 5-33916.
Another is carried out as follows: Waste (the treatable material) is heat-treated in a rotary treatment furnace under low temperature dry distillation so as to convert the treatable material into low temperature dry distillation gas and thermally decomposed residue. The residue is burnt in a high temperature burning furnace thereby forming slag in a molten state. Then, this slag is cooled to be solidified in a glass state. The dry distillation gas emitted from the rotary treatment furnace is supplied to a boiler, or discharged upon treatment by a filter or a gas purifying device. Such a process is disclosed in Japanese Patent Publication (Tokuhyohei) No. 8-510789.
According to the above treatment processes under incineration, the alkali material is sprayed into the incineration furnace, and therefore treatment of noxious gas is made near a location where the noxious gas is generated; however, the treatment is made after the noxious gas has been once generated. As a result, although the removal effect for chlorine-containing gas can be expected to some extent, a sufficient removal effect cannot be so obtained as to meet a recent strict regulation for preventing air pollution. Additionally, such treatment processes are accomplished under incineration and therefore high in reaction temperature. As a result, it is difficult to maintain a stable reaction. In case that a large amount of the alkali material is sprayed, the alkali material affects burning itself so that unburned parts are formed. Thus, it is impossible to meet the recent strict regulation for preventing air pollution.
According to the above other treatment processes under the dry distillation, the treatable material is thermally decomposed without being burnt, so that unstable factors such as incineration furnace and the like can be readily removable. However, the treatment process in which alkali material is sprayed into the furnace will provide the same effects as those in the treatment process under incineration.
In case that waste gas contains a large amount of noxious gases (particularly chlorine-containing gas and sulfur oxides gas), corrosion of the furnace and gas duct is remarkable so that there is the fear of lowering durability of the waste treating facility and raising gas leak, thus making maintenance of the facility difficult.
Further, in case that the waste contains sulfur component, when the waste is incinerated upon adding calcium-containing alkali material such as CaO to react with sulfur oxides gas, CaSO4 (calcium sulfate) called gypsum is formed. Gypsum solidifies upon absorbing water content, and therefore post-treatment for gypsum becomes difficult.
As appreciated from the above, according to any of the above-discussed treatment processes, noxious gas is once generated from the treatable material at a former step, and thereafter chlorine-containing gas, sulfur oxides gas and dioxins are removed at a latter step by means of a bag filter or burning treatment. Accordingly, it is difficult to sufficiently remove noxious gases and dioxins.
In order to solve the above-discussed problems, the following treatment process has also been proposed: When the treatable material is heat-treated in a treating furnace, a suitable amount of alkali material (treatment agent) liable to react with chlorine component is mixed with the treatable material so as to fix the chlorine component in residual ash thereby obtaining harmless waste gas. The residual ash is rinsed with water or the like thereby removing the chlorine component. This process is disclosed in Japanese Patent Provisional Publication No. 9-155326.
In this process, the treatment for thermally decomposing the treatable material to generate decomposition gas is accomplished in a single treating furnace. In other words, a series of steps for supplying the treatable material into the single treating furnace through the supply inlet and for discharging carbonized material from the single treating furnace through the discharge outlet are accomplished in the single treating furnace. In the series of steps, the treatable material is stirred and subjected to heat treatment (for example, for 1 hour and at a temperature ranging from 300 to 600° C.), thereby successively accomplishing the respective steps of drying, thermally decomposing and volume-reducing (carbonizing) the treatable material.
Now, noxious gas containing halogen substances are generated at a temperature ranging from 200 to 350° C. upon thermal decomposition of the treatable material, in which the treatable material and the treatment agent react with each other to form harmless salts. However, there is the possibility that a part of the noxious gas is left in its non-reacted state. Additionally, the treatable material is being stirred, and therefore there is the possibility that the non-reacted noxious gas can be involved in the treatable material. In case that the treatable material is heated at a temperature over 350° C. so as to form carbonized material, the noxious gas will be absorbed in the carbonized material.
When the carbonized material, the noxious gas and produced dioxins produced exist simultaneously, the carbonized material unavoidably adsorbs these noxious gas and dioxins. It will be appreciated that it is very difficult to remove the noxious and dioxins which have been once adsorbed. Accordingly, it is difficult to reuse the carbonized material, and therefore it is required to bury the carbonized material in a final disposal place, or otherwise to cause the carbonized material to be subjected to another treatment such as a fusion treatment at a very high temperature.